The present invention relates to a control system and more particularly to a joystick type control system, and particularly to such systems utilizing magnetic positional sensing used in safety critical human/machine control interfaces.
Typical uses for such control systems include wheelchairs, forklift trucks or other man-carrying vehicles, and control of machines such as cranes, robots or other industrial equipment where a dangerous situation could be created in the event of a control system failure. In such a system, dual joystick position sensor channels may be used, and the outputs compared to one another continuously. This ensures that if there is a problem with one of the sensor channels, the error is identified due to a mismatch in the outputs at the two channels. If a discrepant output (differential beyond a predetermined threshold) occurs, the control system is configured to rapidly and safely disable the system.
Due to the fact that the two sensors in each fail-safe pair cannot occupy exactly the same position in space, and due to the small differences in calibration that will occur between them, the outputs from the sensors in the pair will differ slightly and allowance must be made for this when setting the system permissible differential tolerance threshold. The sensors are typically programmable, allowing each pair to be calibrated to provide a nominally zero difference in output from each sensor of the pair, under normal operating conditions. However, if the threshold is too small then the monitoring system may indicate a malfunction when creating ‘false errors’ or ‘nuisance trips’ as known in the art.
Alternatively, the sensors in each pair could be arranged to provide outputs having opposite sense. In such an implementation, the output of one sensor of the pair could be arranged to provide a positive output, and the other sensor of the pair could be arranged to provide a negative output. In this arrangement, the sum of the outputs of the sensors in a given pair, or their mean, would be required to be a constant to within the tolerance threshold.
For joystick systems of the magnetic sensing type, it is therefore necessary to measure the angular position of the joystick shaft (and therefore the magnet) without introducing errors due to the linear motion of the magnet in the three orthogonal directions.
International Patent Application No. WO 2006/013323 A1 describes a control system comprising a magnet, biased to a central upright position and movable about two perpendicular axes by means of a shaft coupled thereto. The magnet is molded within a ball situated in a socket and the ball is surrounded by a pole-piece frame arrangement which lies in a plane that is substantially perpendicular to the axis of the shaft. The pole-piece frame arrangement typically comprises four pole-piece arms arranged with four respective gaps therebetween, the gaps being equally spaced around the magnet. Within each of the four gaps there is provided a Hall effect sensor such that opposing pairs are arranged to detect either forward/aft or left/right deflection of the shaft. In use, the angular movement of the shaft toward a first gap creates a magnetic potential difference within the pole-piece frame, which causes flux to flow symmetrically around the circuit to the diagonally opposite gap of the pole-piece arrangement. Thus, flux lines will flow from the magnet to the pole-piece frame arrangement and from there to the gaps diagonally opposite to the above-mentioned first gap, passing through a Hall effect sensor located in the gap therebetween, thereby generating a signal to activate the desired control. The input conveyed by the user via the shaft is only actioned if the flux measured in one sensor of a sensor pair is also measured to in the second sensor of the same sensor pair to within a tolerance threshold. This multiple sensing provides a fail-safe in the event that one of the sensors of a sensor pair generates an erroneous signal.